The present invention relates to aerobic fermentation processes, and provides a method and apparatus which is especially suitable in cases where the fermentation products are highly viscous.
A typical fermentation process includes four phases, namely the lag phase, the growth phase, the stationary phase, and the death phase. The lag phase is essentially a preparatory phase, while the remaining phases are operational phases in which the actual fermentation process is conducted.
The operational phases of an aerobic fermentation process require a constant supply of oxygen. Without oxygen, the organisms responsible for the fermentation process may die, or they may produce an undesirable product. Oxygen must be continuously transferred into the fermentation medium, in order for the fermentation process to continue, and for the microbes in the fermentation medium to produce a desired product.
In certain applications, the fermentation products become very viscous. For example, in the manufacture of xanthan gum, using Xanthomonas Campestris, the fermentation broth begins with a viscosity comparable to that of water, of the order of 1 centipoise (cp). But as the process continues, and the concentration of the product xanthan gum approaches 2-4%, the viscosity of the fermentation medium may reach a level of the order of 20,000 cp. By comparison, a typical value for the viscosity of oil is in the range of about 100-300 cp.
The production of a highly viscous fermentation product is made difficult by the fact that the viscosity of the medium inhibits the transfer of oxygen to the organisms in the medium. In the prior art, the oxygen has been derived from a stream of air which is pumped into the fermentation vessel, the air being mixed into the medium by mechanical agitators. But the more product that is made, the more viscous the medium becomes, and the greater the difficulty in transferring the necessary oxygen into the medium. The fermentation process is therefore limited by the rate of oxygen transfer. The process is self-limiting, insofar as the very success of the fermentation process will cause the process to slow down, or even cease, due to lack of oxygen. When the fermentation product is highly viscous, the oxygen transfer rate becomes the most important limiting factor to the productivity of the fermentation process.
In the prior art, efforts have been made to solve the above problem by increasing the flow of air into the fermentation vessel, and increasing the agitation speed. Increasing agitation speed produces more turbulence in the fermentation medium, and the turbulence tends to enhance the mixing of air into the medium, thereby increasing the oxygen transfer rate. But experiments have shown that, while increasing the agitation speed does improve the oxygen transfer rate, the improvement is only marginal. The exact amount of improvement obtained by increasing agitation speed depends on the geometry of the vessel and the configuration of the agitator blades. Moreover, increasing the agitation speed consumes considerable energy, and it turns out that the benefit in increased oxygen transfer usually does not outweigh the additional energy cost.
Likewise, increasing the flow rate of the air introduces new problems, such as impeller flooding, foaming of product in the fermentation medium, and reduction in the efficiency of dissolution of oxygen into the medium, due to a reduced residence time of air in the vessel.
Still another way to address the problem is to replace the fermentation vessel with a larger one. This approach would tend to delay the onset of oxygen starvation, because of the larger volume of the fermentation medium. But this approach is expensive, and it only postpones, and does not solve, the fundamental problem of oxygen transfer through a viscous medium.
The present invention provides a fermentation process and method which solves the problem described above, and which provides an economical means for making a highly viscous fermentation product.